The present invention relates generally to integrated circuit memory devices, and more specifically to integrated circuit memory devices which employ thin-film transistor (TFT) technology.
Thin-film transistors (TFTs) are becoming the load devices of choice in many integrated circuit memory devices, particularly in static random access memory (SRAM) cells. TFTs are superior to standard polysilicon resistor load devices, in that TFTs have an inherently lower OFF current—an advantage which is particularly relevant in low—and zeropower SRAM applications which feature extended battery operation. In spite of this advantage, however, the bitline to supply (Vcc) leakage of TFTs designed and fabricated in state-of-the-art technology is still too significant to enable battery operation of high-density memory devices, such as SRAMs, over an extended period of time.
The most common approach taken to reduce this bitline to supply leakage has been to reduce the cross-sectional area of the TFT channel, such that the TFT channel is made as thin and as narrow as possible. To this end, technologies which are capable of depositing extremely thin polysilicon layers, having a thickness of approximately 100 Å for instance, have been developed. Unfortunately, the resultant polysilicon grain size of these layers is also very small. Alternately, the width of a TFT of a memory cell may be made much smaller than any other critical dimension (CD) in the circuit. Thus, there are currently available products which feature TFTs having channel widths of 0.3 to 0.4 μm wide while all other CDs are 0.5 μm or larger. As would be anticipated, this difference between the width dimension and other CDs of the memory device places considerable pressure on the photolithography aspect of manufacturing and thus makes manufacturing of a device using such geometries very difficult. Additionally, there are processes which fully enclose the TFT channel by the device gate. This results in process complications which do not render a viable manufacturing approach.